Simplify the following expression: $7\sqrt{27}$.
First, try to factor a perfect square out of the radical. $= 7\sqrt{27}$ $= 7\sqrt{9 \cdot 3}$ Separate the radical and simplify. $= 7\sqrt{9} \cdot \sqrt{3}$ $= 7 \cdot 3 \cdot \sqrt{3}$ $= 21\sqrt{3}$